1. Field of the Invention
This invention relates generally to refrigerators and more particularly to cryogenic coolers which employ regenerators.
2. Description of Related Art
Over the past several decades, compact cryogenic refrigerators have been developed to give reliable cryogenic temperatures from about 8.degree. K. to 150.degree. K. in very small spaces. Some of the more successful cryogenic refrigerators today employ thermal regenerators to accomplish the variety of refrigeration cycles such as the Stirling cycle, Split-stirling cycle, Gifford-McMahon cycle, Solvay cycle, pulse tube cycle and the Vuilleumier cycle. Thermal regenerators are typically incorporated into either a displacer or a piston which reciprocates within the particular refrigerator arrangement to accomplish one of these operating cycles.
For example, a conventional regenerative cryogenic refrigerator may have a displacer located within a fluid-tight enclosed chamber. The displacer divides this chamber into two smaller chambers, namely a warm chamber and a cold chamber. Within the displacer is a thermal regenerator which typically has a cylindrically-shaped bore containing a matrix of metallic screens therein, and opening at each end to the warm and cold chambers. Accordingly, gas may pass through the regenerator flowing from one chamber to the other.
In typical operation, the displacer/regenerator reciprocates back and forth within the fluid-tight enclosed chamber varying the volumes of the warm and cold chambers and passing gas therebetween. The cold chamber is the region where refrigeration occurs and is the location upon which devices to be cooled such as infrared sensors are mounted. To cool such devices, a high pressure fluid is introduced into the warm chamber and flows through the regenerator exiting into the cold chamber through a hole at the end of the displacer. The high pressure fluid is cooled as it passes through the regenerator. The displacer moves toward the warm end, increasing the volume of the cold chamber and simultaneously filling the cold chamber with more high pressure gas. Next, the pressure in the warm cold chambers is reduced, and accordingly the gas in the cold chamber is extracted back through the regenerator and exits into the warm chamber at about ambient temperature. The gas in the cold chamber therefore expands reducing the temperature of this gas. The cooled gas absorbs heat at the cold end before passing through the regenerators. Next, the displacer moves toward the cold chamber, decreasing the volume of the cold chamber which still contains low pressure gas. High pressure fluid is again introduced into the warm chamber which passes through the regenerator to the cold volume increasing the pressure in the cold chamber. This increase in cold chamber pressure increases the temperature of the gas therein. However, since more heat is taken from the cold chamber than put into it, a net refrigeration effect takes place in the cold chamber to provide the desired cooling.
Historically, the heat transfer path between the regenerator and cold chamber was accomplished by a hole at the end of the regenerator. The end hole directs the gas exiting the regenerator onto the end wall of the cold chamber. This technique provided efficient heat transfer at the cold end of the refrigerator, and is illustrated in U.S. Pat. Nos. 3,877,239 and 3,913,339, for example, which are assigned to the assignee herein. However, as larger cooling capacity refrigeration arrangements were developed and accordingly as refrigerators and their respective parts increased physically in size, the end hole did not provide for efficient heat transfer to the cold chamber. The end hole was replaced by radial holes located near the end of the displacer as exemplified in U.S. Pat. Nos. 3,218,815 and 3,303,658, for example. In refrigerators employing radial holes, gas exits the regenerator and impinges on the annular inner wall of the cold chamber. The gas is distributed over a larger surface of the cold chamber. Therefore, heat is transferred from the cold chamber walls over a larger area and with a larger heat transfer coefficient than was possible with the end hole.
Today refrigerators are being made smaller and smaller to meet size and weight requirements desired by both military and commercial customers. Furthermore, as miniature refrigerators are increasingly used to cool electronic devices in remote environments, high reliability, high efficiency and long maintenance-free life refrigerators are being demanded. In most present day refrigerators, a major source of loss of performance or failure results from the freezing of condensable contaminants blocking the passageway from the regenerator to the cold end. Although vigorous cleaning procedures, including pumping and baking, may be employed, undesirable amounts of condensable contaminants are still present within the refrigerator after filling with working fluid. Moreover, additional contaminants are generated by chemical reactions between component parts of the refrigerator. Presently no refrigeration arrangement has been realized to effectively deal with these contamination problems which have plagued the industry for many years.